By the way, the proper way to ask this question is "Are all of these sentences grammatically correct?"

Thanks for making my day! They are the same，but a gum job is better because it doesn't involve teeth.

The second is correct because "do you think" is the inverted verb-subject that makes the sentence a question. "The reason is" is the object of the sentence; there is no reason to invert the subject and verb. Americans frequently say the first and, similarly, use noun clauses incorrectly. For example, people sometimes say, "I wonder what is she going to do today?" But a noun clause ("what she is going to do today) is acting as a noun (subject or object) and, therefore, the subject-verb order should not be inverted. The correct way to say this is "I wonder what she is going to do today."

I used to teach an entire class on responding to "What" and "How" greetings. When someone greets you with a "What" question （What's up? What's happening? What's going on?) and it's just a greeting, appropriate responses include: Not much Nothing Same old thing Nothing new For "How" questions (How are you? How are you doing? How have you been?), appropriate answers include: Fine. Pretty good. Not bad. Not too good. The key is whether the greeting is a "what" or a "how" question.

No, certainly not in American English; that would sound weird. Common responses to "Thank you very much" in the States include "You're welcome." "No problem." In the South: "You bet." There are many other phrases used, also, but "It doesn't matter" sounds like you are blowing the person off (do not value their thanks).

Damage and ruin can also be nouns. Destroy is only a verb.